Field of the Invention
The present invention relates to a cover device for a dewatering shoe of a papermaking machine.
More specifically, the present invention relates to a cover device for a dewatering shoe of a papermaking machine in which the cover permits a flow therethrough of water removed from a web.
Background Information
The invention concerns a fluid previous cover device for a drainage shoe or a drainage box, curved vacuum box and the like or a similar drainage device in a papermaking machine. The cover device may also be suitable for use with a suction or gravity drainage box in the forming section, a Uhle box in the press section, or other drainage device used to assist with water removal from a web being conveyed through the machine.
In the manufacture of paper and similar cellulosic products, an aqueous stock consisting of about 97-99% water and 3% to about 1% papermaking solids is ejected at high speed onto a forming fabric in the forming section of a papermaking machine. The forming section is required to drain a large percentage of this water through the forming fabric so that, by the end of the forming section, a nascent web consisting of about 20% to 25% papermaking solids is transferred to a press section where a further proportion of water is removed by mechanical pressure. The resulting web then passes to the dryer section of the machine where the remaining water is removed by evaporative means so that, by the end of the dryer section, the final paper product consists of about 97% to 99% papermaking solids with the remainder as water. Various known drainage devices are used in the forming and press sections of the papermaking machine, such as the initial forming shoe, other drainage shoes (which may or may not be vacuum assisted) and devices such as Uhle boxes in the press section; these devices are required to manage and remove these large volumes of water effectively and efficiently. Such devices, and their covers which arc shaped and located so as to provide contact with the forming or press fabrics and to assist the management of the water drained from the sheet, have been previously described. See for example, Irwin et al. U.S. Pat. No. 4,734,164; Fujiwara U.S. Pat. No. 4,425,188; Wildfong et al. U.S. Pat. No. 6,372,091; Buchanan et al U.S. Pat. No. 7,005,040, and others which disclose various means of water management including grooves, flow through venting. A problem common to all of these prior art solutions is the high cost of manufacturing these drainage box covers, which are typically formed from a ceramic or other wear-resistant surface material that is arranged so as to be in intimate contact with the forming fabrics upon which the embryonic web is located. This surface must be vented, drilled or otherwise machined to provide the necessary flow-through drainage into the drainage device located in supporting contact with the cover. A long felt need has existed in the papermaking art for a cover for a drainage device for use in a papermaking process in which the cover could be quickly and easily manufactured to provide a selectable open area at a much lower cost than has previously been possible. Additionally, there has been a need for the cover device to be flexible in at least one of the machine direction and cross-machine direction to permit production of a cover having a variable curvature. Also, there has been a long felt need for a cover that would be resistant to abrasive wear caused by prolonged sliding contact between the papermaking fabrics and cover surface. The present invention addresses these long felt needs.
The drainage surface of the cover of the present invention has a selectable open area and is comprised of an expanded metal material, in particular a tubular slit expanded metal. The expanded metal is preferably of stainless steel which has been coated with a suitable wear resistant material such as a ceramic or other wear resistant coating.
Expanded metal is a well known and versatile construction material and has been disclosed in for example EP 797,486 to Kloeckner, U.S. Pat. No. 6,691,386 to Marlow and U.S. Pat. No. 7,934,301 to Mulder.
One example of a suitable expanded metal for use in the present invention is of the type referred to as “tubular slit” and is commercially available from Ferguson Perforating Inc. of Providence, R.I. This material is formed from a planar metal sheet which has been regularly and discontinuously slit and then expanded in the Z-direction between each slit to provide a regular grid of generally rectangular openings between which are located planar surfaces which have been deformed above and below the plane of the sheet to form first and second surfaces. The expanded areas form regular, parallel, generally rhombic shaped tubular openings separated by and interconnected by land areas oriented in a first direction across the plane of the sheet. The opposing surfaces of the openings are alternately and equidistantly expanded in a second direction (the Z-direction) so as to be raised above and below the plane of the land areas to provide generally planar opposing surfaces. The opposing surfaces are oriented in a third direction along the plane of the sheet which direction is perpendicular to the first and second directions. The opposing surfaces define the tubular openings and land areas oriented in the first direction.
In the present invention sections of tubular slit expanded metal such as described above are welded or otherwise bonded together in a size suitable to form the surface or cover device of a drainage shoe, suction or other drainage box over which a papermaking or other industrial filtration fabric or dewatering screen and product thereon will pass in sliding contact as they move continuously from an upstream to a downstream edge of the cover device in a machine direction.
When in use, the expanded metal sheet is oriented on the drainage device so that the series of tubular openings and land areas extending in the first direction are oriented at an angle of about 45° to the machine direction. During operation, fluid is skimmed from the underside of the filtration fabric by the first surface which is the fabric contact surface of the expanded metal as the fabric passes between the generally rectangular openings and land areas of the first surface. The fluid then passes through the tubular openings and out through the corresponding openings of the second surface. The open area of the drainage surface is selectable by appropriate selection of the size of the rectangular openings and land areas in at least the first surface of the expanded metal sheet.
The novel drainage surfaces of the present invention offer the advantages of ease of manufacture and a reduction in cost to the consumer.
The novel cover device according to the present invention provides many benefits regarding the manufacture of the cover device and the operation thereof on a paper machine.
Numerous advantages are obtained by forming the cover device of a drainage apparatus from expanded metal. These advantages include:    1/ Expanded metal provides increased stiffness compared with a prior art single sheet drilled or perforated plate.    2/ Expanded metal can create bending flexibility in one direction and stiffness in another direction which was not previously possible; this permits variation in cover device design and the ability to simplify manufacture of a variable curvature cover.    3/ The welded structure of the cover device incorporating expanded metal according to the present invention is easier to manufacture thus reducing the manufacturing complexity of the cover device and delivery time.    4/ The expanded metal structure of the cover device, with the ability to be recoated multiple times, provides a much longer life potential than ceramic drainage device covers which can only be reground typically, once if at all.    5/ The relatively thinner ceramic coated metal material in contact with the fabric allows water to be removed more easily than for example with the prior art covers in which the water removed from the fabric has to pass along surface grooves along or through long drilled holes.    6/ The relatively more open structure of the cover device of the present invention is less prone to plugging problems than previous prior art arrangements.    7/ The present invention provides a novel cover device and the ability to easily shape the fabric contacting surface profile thereof which offers unique opportunities to affect drainage rates and a flow through the cover and thus adapt it to different applications on the machine.    7/ The open area of the cover can also be varied independent of, or as part of the profile of the surfaces, by changing the size of the upper and lower surface contact points.    8/ For certain applications, the open area can be significantly increased while maintaining a high support index for the fabric. This can be particularly important where a prior art slotted cover tends to damage fabrics or where deflection due to high vacuum induces unwanted forces into the stock structure supported on the fabric.    9/ The flexibility of the material in one direction and not the other allows the cover to support itself in the cross machine direction but also to be flexible in the machine direction. This offers the opportunity to build a cover that can be changed in shape during operation to allow drainage rates and pressure pulses to be altered to achieve optimal benefits. This is particularly important in the impingement or initial forming section area of the paper making machine.
Therefore, it is a primary feature of the present invention to provide a cover device for a dewatering shoe of a papermaking machine that overcomes the problems associated with the prior art arrangements.
Another feature of the present invention is the provision of a cover device for a dewatering shoe or the like that greatly reduces the cost of manufacture thereof.
Another feature of the present invention is the provision of a cover device for a dewatering shoe that provides better drainage and flexibility compared with prior arrangements.
Other features and advantages of the present invention will be readily apparent to those skilled in the art by a consideration of the detailed description of a preferred embodiment of the present invention contained herein.